RF filters are required particularly in terminal devices for mobile communications. They are generally designed for use in mobile radio systems, which transmit and receive in frequency bands lying close to one another. The receive filters are typically constructed as bandpass filters, the passband of which corresponds at least to the bandwidth of the RX band. It is also generally required that the frequencies of the associated TX-band be suppressed.
Various technologies are suitable for manufacturing RF filters. Dielectric filters are produced from discrete L and C elements and require a number of discrete elements in order to provide the required filtering properties. Microwave ceramic filters are economical to manufacture, have low insertion loss, but are generally too large for installation in miniaturized terminal devices. Filters constructed on the basis of bulk acoustic wave resonators or surface acoustic wave elements offer good electrical properties. SAW or surface wave elements, which are distinguished by a small size and a great variety with respect to the electrical parameters that can be adjusted, are particularly preferred.
RF filters on a SAW basis can be constructed from a number of resonators as DMS filters (dual-mode SAW) or as reactance filters, which are connected in a serial branch and parallel branches thereto in the form of a ladder-type arrangement or a lattice arrangement.
In addition to small size and easy manufacturability, a given bandwidth is required for an RF filter, wherein the passband should fall off with a steep edge towards the stopband. The insertion loss in the passband should be minimal in order to minimize power losses. On the other hand, a RF filter should be power-durable and in the long term there should be no damage to the electrode structures even with high input and output power.
In order to improve the adjacent-band suppression, the suppression of TX frequencies in a RX filter, for instance, it was already proposed to connect a two-track, three-transducer DMS filter at the input and output to parallel resonators. Such a structure does not prove sufficiently power durable, however, and has a relatively high insertion loss.
A pure reactance filter requires an excessively large number of basic elements, each basic element consisting of a series resonator and a parallel resonator, to achieve sufficient adjacent-band suppression. With an increasing number of basic elements, however, the insertion loss of such filters deteriorates.